ipv6: do not call ndisc_send_rs() with write lock

Because vxlan module will call ip6_dst_lookup() in TX path,
which will hold write lock. So we have to release this write lock
before calling ndisc_send_rs(), otherwise could deadlock.

Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>